An internal combustion engine for a motor vehicle generally includes an engine block defining at least one cylinder accommodating a reciprocating piston coupled to rotate a crankshaft. The cylinder is closed by a cylinder head that cooperates with the reciprocating piston to define a combustion chamber. A fuel and air mixture is cyclically disposed in the combustion chamber and ignited, thereby generating hot expanding exhaust gasses that cause the reciprocating movements of the piston. The fuel is injected into each cylinder by a respective fuel injector. The fuel is provided at high pressure to each fuel injector from a fuel rail in fluid communication with a high pressure fuel pump that increases the pressure of the fuel received from a fuel source. Operation of the internal combustion engine is generally controlled by one or more electronic control units (ECUs) operably coupled to an array of sensors and actuators associated with the internal combustion engine.
Due to stringent emissions regulation, internal combustion engines generally include exhaust gas after-treatment systems. An after-treatment system may include one or more after-treatment devices provided in an exhaust system of the internal combustion engine. For example, an after-treatment system may include an oxidation catalyst such as a diesel oxidation catalyst (DOC), that is, a device that utilizes a chemical process to break down compounds within the exhaust stream turning them into generally harmless components. DOCs may have a honeycomb shaped configuration coated in a catalyst designed to trigger the required chemical reaction. DOCs typically contain palladium (Pd) and platinum (Pt) or cerium oxide, which serve as catalysts to oxidize hydrocarbons and carbon monoxide into carbon dioxide and water. An alternative to DOC may be a three-way catalyst (TWC).
In a further alternative, a lean NOx trap (LNT) may be used. A LNT is a device that traps nitrogen oxides (NOx) contained in the exhaust gas. More specifically, a LNT is a catalytic device containing catalysts, such as rhodium (Rh), Pt and Pd, and adsorbents, such as barium based elements, which provide active sites suitable for binding the nitrogen oxides (NOx) contained in the exhaust gas, in order to trap them within the device itself.
After-treatment systems may also include a diesel particulate filter (DPF) which filters particulate matter (PM) from the exhaust gas and a selective catalytic reduction (SCR) device which is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatomic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3) that can be obtained by urea (CH4N2O) thermo-hydrolysis and that is absorbed inside the SCR. Typically, urea is contained in a dedicated tank and is injected into and mixed with the exhaust gas flow upstream of the SCR. Other fluids can be used in a SCR in lieu of urea, any of which are generally referred to as diesel exhaust fluids (DEF). An alternative to the SCR is a SCRF (SCR on filter), that is, a device that combines in a single unit an SCR and a DPF.
The LNT, whether stand alone or in combination with an SCR, needs periodic cleaning. Cleaning of the LNT occurs in a process called regeneration, which is generally triggered automatically by the ECU when a threshold level of contaminants is detected in the LNT and additional operating parameters of vehicle are present. Regeneration may be provided by switching the engine from lean operation to rich operation, whereby excess fuel usually provided as a fuel after-injection in the exhaust gas is burned in the exhaust system raising the temperature of the exhaust gas.
Successful regeneration requires maintenance of the rich condition until regeneration is complete. Otherwise, the process must be repeated. Rich condition operation may be confirmed by observing a lambda (e.g., air-fuel ratio, oxygen, etc.) sensor output, and when, for example the lambda value is at or above a threshold value. However, during normal transient maneuvers, e.g., normal acceleration, to improve drivability and torque output and to reduce emissions output, such as soot production, the rich operation may be interrupted or may not be maintained as a result of the primary or main fuel injection quantity adjustment necessary to accommodate the transient maneuver.